Photosynthetic organisms use pigments—molecules that absorb selected wavelengths of light—to capture photon energy.

The principal pigment of oxygenic photosynthesis, photosynthesis that releases molecular oxygen, is chlorophyll a. Accessory pigments such as chlorophyll b and carotenoids absorb additional wavelength ranges and can transfer absorbed excitation energy into the photosynthetic system.

Because each pigment has a characteristic absorption spectrum, photosynthetic tissue does not absorb all visible wavelengths equally. Chlorophylls absorb strongly in blue and red regions and relatively weakly in green, so much green light is reflected or transmitted.

Antenna pigments enlarge the effective collecting area

Most chlorophyll molecules are not the molecules that directly initiate electron transfer. They are arranged with proteins in light-harvesting antenna complexes around a reaction center.

When one antenna pigment absorbs a photon, its electronic excitation can be transferred to a neighboring pigment without transferring the excited electron itself between those two antenna molecules. Repeated energy transfer funnels excitation toward the reaction center:

photon absorbed by antenna pigment
        ↓
excitation energy transferred among pigments
        ↓
reaction-center chlorophyll becomes excited

This distinction matters: energy transfer through the antenna precedes charge transfer at the reaction center.

Accessory pigments can also protect the system

Not every absorbed excitation can be used productively. Excess excited-state energy can generate damaging chemistry. Carotenoids and other protective mechanisms can dissipate some excess excitation rather than allow uncontrolled reactions.

A photosynthetic antenna therefore broadens and concentrates useful light absorption while helping manage excitation that cannot be converted productively.

The core principle is that pigments convert a broad light field into localized molecular excitation; the reaction center performs the next step, converting excitation into separated electrical charge.